Switched-mode power supplies (SMPS) are widely used for providing power to loads such as computers, television sets, lighting systems, home appliances and other electronic devices. As with other types of power supplies, an SMPS converts electrical power from a power source into a form more appropriate for a load. In typical applications, the source power is obtained from an alternating current (AC) mains voltage with a frequency of 50 Hz to 60 Hz and root mean square (RMS) voltage levels of 90V to 240V. The SMPS typically converts this source power into a lower-voltage direct current (DC) power that is supplied to the load.
SMPS have several advantages over other power supply types. SMPS are often significantly more efficient than other power supply types, as SMPS waste less energy in ohmic losses, e.g., as done by power supplies relying on linear regulation. Because wasted energy must typically be dissipated as heat, SMPS have reduced heat dissipation requirements meaning fewer and/or smaller heat sinks are required. Furthermore, many SMPS configurations require no transformer, while other SMPS configurations require a transformer that is significantly smaller than is required by other power supply types. For these reasons, SMPS are often smaller and less expensive than other power supply types.
While there are many configurations for SMPS, all of them switch (enable and disable), at some level, power that is ultimately supplied to an output load. An SMPS controller governs this switching by varying parameters such as the frequency and/or duty cycle of a pulse-width-modulated (PWM) signal that is used to drive one or more switch devices, e.g., transistors, such that the output power supplied to the load meets the load's power requirements in terms of voltage and current.
The SMPS controller is, itself, an electronic device that must be powered. During the normal operational mode of the SMPS, the SMPS controller may be powered, e.g., by tapping the same output power that is supplying the load, or by using an auxiliary winding of a transformer within the SMPS. However, such power is only available after the SMPS has achieved steady-state (normal) operation. Some other means for supplying power to the SMPS controller is required during a start-up phase of the SMPS. One solution is to use an external power supply, but this requires extra circuitry, which undesirably increases the cost and size of the SMPS. Hence, techniques in which the SMPS self-starts its controller are preferred.
Many self-starting SMPS use an input DC power, e.g., on the primary side of an isolated (transformer-based) SMPS, to charge the power supply for the SMPS controller during the start-up phase of the SMPS. The SMPS controller power supply, henceforth denoted VCC, is typically coupled to a capacitor, denoted CVCC, which serves to smooth the power supply voltage during normal SMPS operational mode, and which may store energy for powering the SMPS controller during the start-up phase. In a simple solution, a start-up resistor connects an input DC power node to the SMPS controller power supply VCC and its capacitor CVCC. During the start-up phase of the SMPS, current flows through the start-up resistor, and the voltage at VCC rises until it reaches an acceptable operational level for the SMPS controller, e.g., 16V. Once this occurs, the SMPS controller may begin normal operation.
However, the simple solution making use of a start-up resistor has two related problems: start-up speed and standby power consumption. If the resistance of the start-up resistor is relatively small, then the SMPS power supply VCC reaches an operational level quickly, but the SMPS standby and/or light-load power consumption is high due to the ongoing power loss in the start-up resistor. Conversely, a start-up resistor with a large resistance leads to lower standby and light-load power consumption, but the SMPS start-up time is high.
An improved solution that addresses these problems makes use of a dedicated power switch, sometimes termed a start-up cell, to supply current from the input DC power to the SMPS controller power supply VCC and its capacitor CVCC during the start-up phase of the SMPS. Yet another improved solution re-uses an existing power switch of the SMPS power stage, in a cascode configuration, rather than requiring a dedicated (separate) switch. Each of these solutions provides high current to the SMPS power supply VCC during the start-up phase, thereby providing a fast start-up time, but do not incur high standby power losses because current does not unnecessarily flow through the power switch during light-load or standby conditions. Use of a cascaded power switch for charging the SMPS power supply VCC requires that the power switch be controlled such that it is enabled (conducting) during the start-up phase, i.e., before the SMPS controller is operational, but that the powerswitch be disabled (not conducting) when current is not needed.
Circuits and methods for supplying power to an SMPS controller during a start-up phase of the SMPS are desired.